A fiber-reinforced composite material comprised of an epoxy resin or other resin reinforced by glass fiber, carbon fiber, or another high strength and high elasticity fiber (glass fiber-reinforced plastic or carbon fiber-reinforced plastic) is particularly superior in mechanical properties, so is being used in broad applications from fishing rods, golf club shafts, and other sports applications to aerospace applications and general industrial applications. As the method for forming such fiber-reinforced composite materials, the method is known of using an intermediate material called a “prepreg” comprised of a reinforcing fiber impregnated in advance with a resin composition forming a matrix resin to form a sheet.
In this respect, in use of a reinforcing fiber, there are the inconveniences of the material being low in elongation and fragile, the orientation of the fiber being disturbed, and the fiber separating and becoming loose. Further, in general, the bondability with the matrix resin is poor, so in a fiber-reinforced composite material, it is difficult to sufficiently realize the superior properties of the reinforcing fiber. Therefore, in the past, the reinforcing fiber has been treated with a sizing agent. Such a sizing agent is used to improve the handling ability of the reinforcing fiber and, further, improve the bondability with the matrix resin to improve the properties of the fiber-reinforced composite material, so has the functions of improving the ability of the matrix resin to impregnate the reinforcing fiber and, further, improving the bondability of the matrix resin and the reinforcing fiber etc. As such a sizing agent, various ones are known, but for example, Patent Document 1 discloses a sizing agent for carbon fiber which contains a methacryl group and cyclic aliphatic epoxy group. Such a sizing agent contributes to the improvement of the resin impregnability not only for an epoxy resin, but also in particular for an acryl resin, unsaturated polyester resin, vinylester resin, or other radical polymerization type resin and of the bond strength of these resins with the carbon fiber and can give a stable effect of improvement of the physical properties to carbon fiber etc.
On the other hand, Patent Document 2 describes that if such a sizing agent is deposited on carbon fiber, the polymerization of a norbornene-based monomer is obstructed and a semicured state is difficult to achieve when made into a prepreg, furthermore, strength is not manifested even after heating and curing, and a polymer comprised of a norbornene-based monomer with generally little polarity is bonded with carbon fiber by an anchor effect, so it is preferable that there be no intermediate layer made of extra sizing agent between the carbon fiber and the matrix resin constituted by the polymer. Patent Document 2 proposes, for example, to use acetone to wash off the sizing agent so as to reduce the amount of deposition on the carbon fiber to 1 wt % or less, then use the carbon fiber to produce a prepreg. That is, Patent Document 2 teaches that when using a polymer comprised of the generally low polarity norbornene-based monomer as a matrix resin to impregnate carbon fibers to obtain a prepreg for use in producing a fiber-reinforced composite material, the amount of the sizing agent which is deposited on the carbon fiber should be reduced as much as possible.
Further, it is known that when using reinforcing fiber on which a sizing agent is deposited so as to produce a fiber-reinforced composite material, bubbles occur due to the sizing agent at the time of heating and voids are formed in the fiber-reinforced composite material.